Generally gear mechanisms such as a trapezoidal thread worm gear mechanism or a rack and pinion gear mechanism have been used as a mechanism to convert a rotary motion of an electric motor to an axial linear motion in an electric linear actuator in various kinds of driving sections. These motion converting mechanisms involve sliding contact portions. Thus, power loss is increased. Accordingly, size of electric motor and power consumption are increased. Thus, the ball screw mechanisms have been widely adopted as more efficient actuators.
In prior art electric linear actuators, an output member, connected to a nut, can be axially displaced by rotationally driving a ball screw shaft. This forms a ball screw with use of an electric motor supported on a housing. Generally, friction of the ball screw mechanism is very low. Thus, the ball screw shaft tends to be easily reversely rotated when a pushing thrust load is applied to the output member. Accordingly, it is necessary to hold the position of the output member when the electric motor is stopped.
Accordingly, an electric linear actuator has been developed with a brake for the electric motor or a low efficient mechanism, such as a worm gear, is provided as a power transmitting mechanism. In FIG. 8, one representative example is shown. This electric linear actuator 50 has a ball screw mechanism 53 with a ball screw shaft 51 rotationally driven by an electric motor (not shown). A cylindrical ball screw nut 52 is threadably engaged with the ball screw shaft 51, via balls (not shown). Rotation of a motor shaft (not shown) of the electric motor causes rotation of the ball screw shaft 51 connected to the motor shaft. This further causes a linear motion (motion in left-right directions in FIG. 8) of the ball screw nut 52.
The ball screw shaft 51 is rotationally supported on cylindrical housings 54, 55 via two rolling bearings 56, 57. These bearings 56, 57 are secured in position by an anti-rotation member 59 to prevent loosening of the bearings 56, 57 via a securing cover 58.
A helical screw groove 51a is formed on the outer circumference of the ball screw shaft 51. The ball screw nut 52 is threadably engaged with the shaft 51, via balls. A helical screw groove 52a, corresponding to the helical screw groove 51a of the ball screw shaft 51, is formed on the inner circumference of the ball screw nut 52. A large diameter portion 60 is also formed on one end of the nut 52.
A flat portion 61 is formed on the side of the large diameter portion 60 by cutting. It has a flat end face and a cam follower 62 or anti-rotation mechanism for the ball screw nut 52. A rolling bearing projects radially outward from a substantially central portion of the flat portion 61.
As described above, since the cam follower 62 is fit in the cut-out portion, accompanying rotation of the ball screw nut 52 to the rotation of the ball screw shaft 51 can be prevented. Thus, the cam follower 62 rotationally slides on the cut-out portion. Problems of sliding friction as well as wear can be reduced. See, JP2007-333046 A
In the prior art electric linear actuator 50, it adopts the cam follower 62 as the anti-rotation mechanism for the ball screw nut 52. Thus, it is possible to reduce problems of sliding friction as well as wear. This reduces operating torque of the electric linear actuator 50. However, the cam follower 62 itself uses the rolling bearing. This increases manufacturing cost and any anti-wear measures when the housing 54 is formed from aluminum.